Fix negative damage from Zombies. Fixes BUKKIT-4193

EntityZombie.getMaxHealth() returns a hardcoded value of 20, which is the
vanilla max health for zombies. Rather than using this value when calculating
zombie damage, the call is changed to instead use ((CraftLivingEntity)
this.bukkitEntity).getMaxHealth(). This uses the true maximum health of the
Entity. "this.maxHealth" could be used instead of the aforementioned method,
however that creates a very unclear diff, and a confusing change.